This invention relates to ground anchorages.
A description of the use and construction of ground anchorages is to be found in the Code of Practice for Ground Anchoragesxe2x80x94BS8081xe2x80x94published by the British Standards institution. This Code also contains suggested terminology.
A typical ground anchorage comprises a bore in the ground which is filled with grout (the bore grout). Received in the bore is a tendon generally of steel which at its end remote from the open end of the bore is bonded in an encapsulation comprising a corrugated duct filled with resin or cement grout, to protect the tendon against corrosion. The encapsulation is bonded in the bore grout and the tendon has a free length which is greased and sheathed and has substantially no adhesion to the bore grout. The sheath enters into the encapsulation so that the tendon bond length in the encapsulation is somewhat less than the encapsulation length. At its free end the tendon is received in an anchor head, which is stressed against an anchor plate bearing on the ground and the load on the tendon is locked off against the plate.
The above description relates to a ground anchorage comprising a single tendon. Ground anchorages are also known which comprise a number of steel tendons which are bonded in a single encapsulation of elongate form. The ends of the tendons may be arranged in staggered relation within the encapsulation to spread the load along the encapsulation.
GB 2223518 describes a single bore multiple anchorage comprising a plurality of unit anchorages, each having a tendon, the tendons being bonded in respective encapsulations at staggered and spaced positions along the bore.
This arrangement allows each respective unit anchorage to be loaded to the maximum local capacity of the ground. The total capacity of the multiple anchorage is the sum of the capacities of the unit anchorages. It is very important that the free length of each unit anchor has substantially no adhesion to the bore grout. If this were not the case, there would be a tendency for each unit anchor to apply load in an uncontrolled manner through the bore grout to the ground in the region of encapsulations of other unit anchorages which are higher up the bore. If such uncontrolled or unpredictable loading of the bore grout occurs, it will be impossible to load all of the unit anchors to their maximum capacity without risking failure of the grout to ground bond.
In all of this prior art, an encapsulation is used because the tendons are made of steel, the material of choice due to its strength and availability. The encapsulation serves to transmit the load applied to the tendon to the surrounding grout but at the same time protect the bonded length of the tendon from ingress of moisture which would lead to excessive corrosion and failure of the steel tendons.
In practice, the corrosion protected tendons have to be manufactured off site to a high specification, with the bonded length grouted into the encapsulation in the factory.
The present inventor has realised that great flexibility and simplicity can be obtained in a single bore multiple anchorage if tendons made of synthetic polymeric material can be used. In this case, no encapsulation will be required to protect the bond length of the tendon, as it will not be subjected to degradation due to corrosion.
Accordingly, the present invention provides ground anchorage means comprising a bore in the ground filled with bore grout and a plurality of unit anchorages received in the bore. Each unit anchorage comprises a tendon having (a) a bond length which is bonded along the bond length within the bore grout, (b) a free length arranged so that there is substantially no adhesion between the free length and the bore grout, and (c) a gripped length. The bond lengths of the respective unit anchorages are anchored in the bore grout in a staggered and spaced relationship along the bore. At least one of the tendons is of a synthetic material being gripped in the gripped length by a grip arranged to apply compression to the tendon in a direction transverse to the longitudinal direction of the tendon. The compression increases over the gripped length of the tendon in such a manner that, at no point along its length, is the tendon subjected to shear which is greater than the shear strength of the tendon.
Preferably, all of the tendons comprise synthetic polymeric material.
It is surprising that synthetic polymeric material tendons can be used successfully in an anchorage in this way.
The person skilled in the art is aware that the synthetic polymeric material will not bond to the grout as well as steel. This would raise the problem that the bond length of each respective unit anchor would be so long that it would not be practical to accommodate multiples of such unit anchorages in a single borehole. It has been found that suitable surface deformation of the bond length of the polymeric material tendons can result in acceptable bond lengths without substantially reducing the tendon pull out capacity.
Accordingly, to increase the bond of a tendon within the grout, each tendon is preferably profiled deformed on its outer surface within the grout. The surface of the tendon in the bond length be roughened.
Further, the person skilled in the art is aware that polymeric materials which are suitable for manufacturing tendons are very strong in tension but, compared to steel, relatively weak in compression and shear. The person skilled in the art would understand that the gripping arrangements normally provided for steel anchors could not be used with polymeric tendons, as there would be a tendency for the tendons to be crushed by the gripping means. However, the present inventor has discovered that as long as the grip is applied to the tendon in such a manner that compression is applied to the tendon, the compression increasing along the length of the tendon in a manner that, at no point along its length, is the tendon subjected to shear which is greater than the shear strength of the tendon. For example, the gripping force may be spread over a sufficient length and increased in small steps or.at a low rate over a comparatively long length of tendon. Sufficient gripping force can then be applied to the tendon without damaging it and to allow tendon capacities in excess of 50 kN to be achieved.
In particular, the ground anchorage means according to the present invention may comprise gripping means configured to apply compression to the tendon in a direction transverse to the longitudinal direction of the tendon, the compression increasing along the length of the gripped part of the tendon so that at no point is the tendon exposed to a shearing compression which is greater than the compressive shearing strength of the polymeric material. The gripping means may comprise at least two gripping parts for gripping the tendon therebetween, the gripping parts being urged together by a plurality of urging means located at different positions along the gripped length of the tendon.
Preferably, there are at least four, more preferably at least six urging means. Eight urging means have been found to be suitable. The urging means suitably comprise nut and bolt arrangements, for example pairs of nuts and bolts on either side of the tendon.
The bond length of each of the unit anchorages within the bore will be selected depending upon the ground strength, the soil grading and the bond capacity of the grout with the soil/ground at the respective depth.
The tendons may comprise lengths of polymeric fibre, such as nylon of a suitable grade. Most preferably, the tendons comprise composites comprising synthetic polymeric material. For example, they may comprise nylon or kevlar strands embedded in a synthetic resin. Alternatively, they may comprise glass reinforced plastic, or carbon fibre reinforced plastic.
The tendons may be of any suitable shape or any suitable dimensions. The tendons are suitably approximately circular in cross section, preferably having a diameter in the range 10-50 mm. Alternatively, flat cross sections, such as rectangular or elliptical cross sections may be used. Such flat cross sections may have a thickness (minor axis) in the range 3-15 mm and a width (major axis) in the range 20-100 mm. The modulus of the elasticity of the tendon is preferably in the range 50-200 kN/m2. Commonly available tendon materials have moduli of elasticity in the range 50-100 kN/m2.
The strength of the tendons should be high as possible. Preferably, the capacity is at least 50 kN. Typical glass reinforced plastic tendons have a capacity in the range of 50-500 kN. Carbon fibre tendons may have a capacity in the range 2000-3000 kN.
A tendon for use in the present invention may typically comprise a plurality of fibres aligned with the length of the tendon, the fibres being retained in a resin medium. Such tendons are suitably manufactured by a pultrusion process, as is well known to the person skilled in the art. The tendons may be solid or hollow. Hollow tendons may have a central space whose dimensions are in the range 10-30% of the corresponding outside dimensions of the tendon. For example, a 22 mm diameter tendon may have a central hole of diameter 5 mm.
Preferably, the bonding between the bond length and the bore grout is the sole anchoring effect within the bore grout and operates in the absence of any transverse mechanical stop member within the bore grout.
A particular advantage of the invention is that the amount of pre-preparation of the tendons is lower than is required for steel tendons received in grouted encapsulations. The synthetic polymeric tendons may be supplied in straight lengths or wound on to drums or stored in similar ways and inserted into the bore after suitable treatment of the free length. Preferably, however, the synthetic polymeric tendons are not excessively twisted or bent during storage or during placement in the bore, as twisting can expose the tendons to shearing which may damage them.
According to the present invention, there is substantially no adhesion in the free lengths between the bore grout and the tendons. Preferably, there is substantially no friction between the tendons and the bore grout.
The free length is preferably suitably treated to ensure that there is substantially no bond or adhesion and no friction between the free length and the bore grout. For example, it may be lubricated, for example greased, using grease of the type which is known to the person skilled in the art. It may additionally or alternatively be sheathed with plastic material to prevent adhesion to the bore grout.
Portions of tendons adjacent and parallel to bond lengths of adjacent tendons may be surrounded with compression-resistant ducting, for example a tube of rigid material that is strong in a direction transverse to its length.
The force exerted on the grout by the bonding of the tendon acts in a direction to burst the surrounding grout. If the bond length lies in the vicinity of one or more free lengths of adjacent tendons, there may be a problem. Tendons are greased for movement and are not bonded to the bore grout. They accordingly represent regions of weakness in the resisting of the bursting force. This weakness is made worse, if, as is typically the case, the tendons are individually sheathed with one or more layers of synthetic polymeric material covering at least those portions of the tendon near the respective bond length of another tendon. Suitable compression resistant ducting may comprise compression resistant polymeric material or the like.
A similar arrangement is shown in GB 2260999 in relation to a single bore multiple anchorage in which a plurality of metal tendons are held within encapsulations at staggered and spaced positions along the bore.
The ground anchorage means will include an anchor head at the open end of the bore. Each tendon may be received in a separate hole in the head or in a common hole and gripped by gripping means as set out above. The gripping means serves to transfer load from the tendons to the anchor head. The tendons are stressed and locked off in relation to the anchor head preferably separately from one another.
In the method of the invention, each separate tendon may be provided with a respective stressing jack for extending and placing the tendon under load. Each respective stressing jack will extend by a different amount to the other jacks, depending upon the corresponding elastic length of the tendon in the bore. The tendons may be simultaneously loaded to the same load or they may be loaded to different predetermined load.